Phase transition at 434 K, independent strain coupling in second transition at 400 K, and thermal expansivity in ferroelastic K2TeBr6

X-ray diffraction and specific heat measurements have identified two independent strain-coupled processes in a broad phase transition at about 400 K and a normal lambda-type phase transition at 434 K in K2TeBr6 between 20 K and decomposition at 590 K. Phase I is cubic Fm3m with four K2TeBr6 per unit cell, a435=10.7456(14) Å, and expansion coefficients to third-order of α=0.134(3)×10−3 Å K−1, β=0.198(17)×10−6 Å K−2, and γ=0.162(27)×10−9 Å K−3: it transforms on cooling to phase II at 434 K with an entropy change of 1.3 J mol−1 K−1. Phase II is tetragonal, P4/mnc, with two K2TeBr6 per unit cell, a410=7.5755(10) and c410=10.7702(22) Å, with expansion coefficients to third-order for the tetragonal a axis identical to those of the cubic a axis, and linear coefficient αc=−0.103(12) ×10−3 Å K−1: it transforms on cooling to phase III at about 400 K with an entropy change of 2.9 J mol−1 K−1. Phase III is monoclinic, P21/n, with two K2TeBr6 per unit cell, a295=7.4908(10), b 295=7.5492(7), c295=10.6984(11) Å, β 295=90.307(6)°, with monoclinic a axis expansion coefficients to third-order identical to those of the cubic a axis, linear coefficient αb =−0.308(6)×10−4 Å K−1, second-order coefficients αc=0.711(14) ×10−4 Å K−1, βc =0.290(99)×10−7 Å K−2. The monoclinic angle follows a Landau critical power law with β−90∝(400−T)1/2, whereas a−b∝(359−T). Group theoretical analysis of the phase II symmetry operators predicts two independent soft Eg modes for the K and Br atom special positions at the transition to phase III, leading to independent proportionality constants for the thermal variation of β and a−b, as observed. By contrast, the transition from phase I to phase II has only a single soft A2g mode, leaving the K atom special position unchanged. A simple model predicts 1.52 J mol−1 K−1 for the phase transition at 434 K and 2.64 J mol−1 K−1 for that at 400 K. Dynamical short-range tetragonal order may set in above 359 K. Ferroelastic transformation in phase III at room temperature is shown to be accompanied by atomic displacements no larger than about 0.6 Å as the a and b axes are exchanged under applied compressive stress.